Dietary omega3 fatty acids have been associated with higher rates of energy expenditure, lower rates of weight gain and/or lower rates of fat deposition in rodents as well as human infants and adults. These fatty acids also have been shown to inhibit transcription of genes encoding rate-limiting enzymes of lipogenesis and to enhance transcription genes encoding rate- limiting enzymes for mitochondrial and peroxisomal oxidation as well as the skeletal muscle content of the putative thermogenic uncoupling protein (UCP)-2. Studies are proposed to determine if the metabolic and genomic events occur in a concerted way in vivo to increase energy expenditure and, hence, decrease rates of fat deposition and weight gain as well as to explore specific cellular and nuclear mechanisms by which the effects of omega3 fatty acids are mediated. Clinical studies, to be conducted in 3-6-month-old infants scheduled for surgical repair of inguinal hernia, will address the hypothesis that omega3 fatty acids regulate gene expression so as to decrease a rate-limiting enzyme of triglyceride synthesis (i.e., glycerol-3-phosphate acyltransferase), increase the rate-limiting enzymes of mitochondrial (i.e., carnitine palmitoyl transferase) and peroxisomal fatty acid oxidation (i.e.,acyl-CoA oxidase) and increase abundance of UCP-2 and/or-3 resulting in greater energy expenditure secondary to the inefficiency of enhanced peroxisomal and/or uncoupled mitochondrial oxidation. Infants will be assigned randomly and blindly to formulas that differ only in alpha-linolenic acid (either 1 percent or 4 percent of total fatty acids) and muscle will be obtained approximately two weeks later, during the scheduled operation, for assay of the mRNA abundance of the cited transcripts. Energy expenditure will be determined at the same time by indirect calorimetry and correlated with mRNA abundances. Studies to be conducted in rats will examine the hypothesis that omega3 fatty acids decrease production of malonyl-CoA, an inhibitor of carnitine palmitoyl transferase, by decreasing expression of and/or inactivating the rate-limiting enzyme for its synthesis, i.e., acetyl-CoA carboxylase. Other studies in rats will utilize DNAse hypersensitivity and in vivo footprinting assays to identify DNA regions in the UCP-2, UCP-3 and acetyl-CoA oxidase genes that are targets for omega3 fatty acid regulation. In toto, the proposed studies will identify molecular and cellular mechanisms by which omega3 fatty acids govern the expression of genes encoding key enzymes of lipid metabolism and the extent to which these effects impact whole body energy expenditure and, hence, rates of fat deposition and weight gain.